1. Field of the Invention
This invention relates to a fuel supply control system for internal combustion engines, and more particularly to fuel supply control applied during transient operating conditions of the engine such as acceleration and deceleration.
2. Prior Art
Conventionally, it has been generally employed to detect accelerating/decelerating conditions of an internal combustion engine, based upon an amount of change in the opening of a throttle valve and increase an amount of fuel supplied to the engine (fuel supply amount) at acceleration of the engine and decrease the same at deceleration of the engine.
It has also been generally employed to carry out feedback control of a fuel supply amount in response to an output from an oxygen concentration sensor arranged in an exhaust system of an internal combustion engine. The feedback control is also carried out at acceleration and deceleration of the engine.
However, the incremental value and decremental value by which the fuel supply amount is increased and decreased at acceleration and deceleration of the engine are previously set in accordance with change amounts in the throttle valve opening or the like. As a result, the air-fuel ratio of a mixture actually supplied to the engine can temporarily deviate from a desired value, depending upon variations in operating characteristics between engines and/or the properties of fuel used such as gasoline. If the feedback control responsive to the output from the oxygen concentration sensor is carried out with high responsiveness, such deviation in the air-fuel ratio might be corrected. However, in actuality, there is a significant time lag from the time the mixture is burned within a cylinder of the engine to the time the resulting combustion gas reaches the oxygen concentration sensor when a change in the oxygen concentration in the combustion gas is detected, making it difficult to completely correct the deviation in the air-fuel ratio.
For example, let it be assumed that the throttle valve is suddenly opened such that the throttle valve opening .theta.TH changes as indicated in FIG. 1 (a). Then, the output from the oxygen concentration sensor does not cause a change in the fuel supply amount until after a time point t1 when a change in the output from the oxygen concentration sensor causes a change in the fuel supply amount. As a result, the air-fuel ratio of the mixture first changes toward a leaner side and then toward a richer side after the time point t1. This results in degradation in exhaust emission characteristics of the engine as well as in drivability thereof. In FIG. 1 (b), symbol TD1 represents a time lag from the time the mixture is burned within a cylinder of the engine to the time the resulting combustion gas reaches the oxygen concentration sensor when a change in the oxygen concentration in the combustion gas is detected.